This invention relates to cellular mobile telephone systems, and in particular to such systems of the type in which the mode of operation of a mobile telephone handset, and the services available to it, are varied according to the cell in which the handset is located. In a cellular radio network, mobile telephone handsets are capable of making and receiving telephone calls by way of a radio connection with any one of a network of radio base stations, connected to a switching centre and thus to the rest of the telephone network. On call set-up the network identifies the location of the handset, (as will be described in more detail with reference to FIG. 2) and establishes radio communication between the mobile handset and the base station providing the handset with the best quality radio connection. (This is generally the nearest base station, but differences in transmitter power, directionality of antennas, and local topography may cause a more distant base station to be selected). A geographical zone can therefore be defined for each radio base station, within which any mobile handset will generally communicate with that base station rather than with its neighbours. This zone is known as a xe2x80x9ccellxe2x80x9d.
Proposals have been made, for example in International Patent Specification WO97/13387 (Telia AB), for so-called xe2x80x9cHome Zonexe2x80x9d services. In such services the user is charged at a reduced rate (typically equivalent to the tariff for making fixed-line calls) when making outgoing calls from a mobile handset operating within a designated xe2x80x9cHome Zonexe2x80x9d, usually defined as one or more cells of the cellular system. In other words, when calls are made through a specified radio base station (or one of several such radio base stations) a reduced tariff operates. Outgoing calls made from a cell outside the Home Zone are charged at normal cellular rates. The user may be given an audible or visual indication on his handset as to whether the current location of the mobile handset is within the xe2x80x9cHome Zonexe2x80x9d.
Incoming calls (calls made to the mobile handset) are made, as for any cellular mobile telephone, using its directory number, known as its MSISDN (Mobile Station Integrated Services Digital Network) number. All such incoming calls would still be charged to the calling party at the normal tariff for making calls to cellular telephones, whether or not the handset is within the xe2x80x9cHome Zonexe2x80x9d.
This service allows a user to use his mobile telephone within a defined area (typically, at or near his home), paying call charges similar to those charged for use of a fixed telephone. This gives a service, which, from the subscriber""s viewpoint, is similar to a dual mode cellular/cordless, telephone, but without the need to provide a dual-mode handset or a dedicated cordless base station. (A cordless telephone is a radio handset arranged to communicate with a dedicated radio base station connected to a fixed line). The user may indeed choose to dispense with a fixed line, saving himself (and the telecommunications service provider) the cost of its provision.
However, although convenient to the subscriber himself, such an arrangement requires callers to that subscriber to make a call to his MSISDN number, which is charged to the calling party at a call-to-cellular tariff. These tariffs are generally expensive.
Arrangements have been proposed in which the mobile handset can be contacted using either the MSISDN or a xe2x80x9cvirtualxe2x80x9d fixed-line number. This xe2x80x9cvirtualxe2x80x9d number appears to the caller, and to the caller""s billing system, as an ordinary fixed line, but it is not associated with a physical fixed line connection. Instead, there is an association in the switching network between this number and the MSISDN, such that incoming calls made to the virtual fixed-line number are translated to the MSISDN number. Effectively, there is a permanent call-forwarding arrangement from the virtual fixed-line number to the MSISDN number.
The calling party can therefore use the fixed line number to contact the subscriber on his mobile handset, with the advantage of only being charged at a fixed line tariff. If the mobile handset is within the Home Zone there is no extra charge to the subscriber. If the mobile user is outside the Home Zone the call may still be sent to the mobile handset, with an additional charge for accepting calls when outside the Home Zone being charged to the mobile subscriber""s account. The mobile subscriber may instead arrange for incoming calls to be forwarded to an answering service service. In this arrangement the calling party always perceives a service and tariff similar to that which would be provided if there were a real fixed-line connection, with calls forwarded to the mobile user (the called party) at his own expense when away from the Home Zone. Whilst in the Home Zone the mobile user also perceives a service and tariff similar to that which would be provided if there were a real fixed-line connection, and bears the additional costs of both incoming and outgoing calls when outside that zone.
This arrangement is in some ways analagous to the xe2x80x9croamingxe2x80x9d arrangements provided to allow a subscriber to one cellular network to use his telephone in an area served by another network: the home network corresponding to the xe2x80x9cHome Zonexe2x80x9d.
A common feature of many domestic fixed and cordless telephone installations is the provision of multiple handsets, allowing calls to be made and answered in any part of a building, and allowing two or more members of the household to take part in the same call. It is desirable to be able to provide this capability in the Home Zone service described above. In particular it is desirable to arrange that the virtual fixed-line capability is not lost if one of the handsets is taken out of the Home Zone. This allows calls to be made between the mobile user and another member of the household who may have remained at home.
A first aspect of the invention provides a mobile telephone network operating system for controlling the operation of a mobile telephone network having a plurality of mobile handsets and a plurality of base stations for serving the mobile handsets, the operating system comprising means for identifying the base station currently serving a mobile handset, and means for selectively routing incoming calls to the mobile handset or to another destination according to the location of the handset, wherein a plurality of handsets are associated with a single directory number, and a corresponding set of one or more radio base stations is associated with the directory number, and the network operating system comprises routing means for selectively routing call attempts to the directory number to one or more of the said plurality of handsets currently being served by radio base stations in the said set.
Another aspect provides a method of operating a mobile telephone system, the mobile telephone system having a plurality of mobile handsets and a plurality of base stations for serving the mobile handsets, and wherein there are a plurality of handsets associated with a single directory number, the method being characterised in that a predetermined set of the plurality of radio base stations is associated with the directory number and incoming calls to said directory number are selectively routed to one or more of the said plurality of handsets currently served by radio base stations in the set.
The set of base stations may comprise a single base station. Calls may be diverted to another destination if no handset is currently served by the predetermined set of base stations. This other destination may be an answering system, a predetermined divert number, or one or more of the mobile handsets.
The service can replace a fixed line telephone with two or more mobile handsets, which can each act as the fixed line telephone for both incoming and outgoing calls even if the other is absent or switched off. Although the service can emulate a fixed line telephone by defining the same xe2x80x9cHome Zonexe2x80x9d (set of base stations) for each handset, the invention also covers arrangements in which each of the handsets has a different Home Zone specified for it.
Each handset may be allocated a unique priority rating, the routing means being arranged to route calls only to the handset having the highest priority rating, of those currently served by the base stations defining its Home Zone. The relative priority ratings of the handsets may be changeable by the users.
This invention extends the benefits of the Home Zone service described above by adding one or more extra mobile handsets, each accessible using the same virtual fixed-line directory number. In particular it can be arranged for calls to the common directory number to be diverted only if none of the handsets are within the Home Zone, thereby avoiding unnecessary diversion costs.
A telephone system operating according to the invention requires a directory number common to all the handsetsxe2x80x94the xe2x80x9cvirtualxe2x80x9d fixed line number. This common number will typically be in a number series used for conventional fixed lines, so that calling parties can be billed at a standard fixed-line tariff, instead of the generally more expensive cellular tariffs.
In the preferred embodiment to be described, each mobile handset also has its own directory number (MSISDN). Each mobile handset therefore has its own mobile handset identity, but they share a common fixed-line identity. Therefore, a calling party can contact the users of the service either by using the mobile handset identity directly, or by using the common number. If the individual mobile handset number is used, that particular mobile handset will ring, wherever it is, (or the call will be diverted if that handset has call diversion set), and the calling party is charged the normal tariff for making a call to a mobile handset. However, if the common number is used, the call will be sent to one of the handsets currently in the Home Zone. If none are available in the Home Zone, the call may be diverted, but the calling party will still be charged at the reduced tariff, irrespective of the location of the mobile handsets. The additional cost of any such diversion would be borne by the called party, as is normal for such diversions. Effectively, any mobile handsets within the Home Zone act as substitutes for the fixed line telephone. Diversion takes place if no handset within the Home Zone answers, just as it would if a fixed phone does not answer.
Each handset, on being paged, may be identified as being currently in one of three conditions:
unreachable (e.g. out of radio contact or switched off)
in the Home Zone, that is, served by one of the base stations defining the Home Zone;
served by a base station other than one defining the Home Zone
Note that for a handset which is xe2x80x9cunreachablexe2x80x9d, its physical location within or outside the Home Zone does not need to be identified. The paging process does not identify, at this stage, whether a mobile handset is in use (xe2x80x9cbusyxe2x80x9d) and therefore unable to receive a call.
In the prior art system previously described, with only one handset, the single mobile handset is either reachable and within the Home Zone, or it is not. However, the introduction of second and further mobile handsets introduces further possibilities. In particular, two or more of the mobile handsets may be reachable within the Home Zone at the same time, or one or more handsets may be reachable within the Home Zone whilst one or more others are outside, or switched off.
In the embodiment to be described, a hierarchy of handsets is established, and the call is directed only to that handset, of those currently reachable within the Home Zone, that is highest in the hierarchy. Other solutions to the selection of which handset or handsets to call are also envisaged, however.